There is ongoing progress toward widespread replacement of petroleum with bioethanol as an alternative fuel. Bioethanol is produced mainly through saccharification and fermentation of sugarcane or corn. In recent years, a technique is being developed to produce bioethanol from wood-based biomass and plant-based biomass (which are also referred to as cellulosic biomass) such as wood waste or unused portions of crops such as rice straw, which do not compete with foods and feeds.
In order to produce bioethanol from cellulosic biomass as a raw material by a conventional ethanol fermentation method, it is necessary to saccharify the cellulose. As a saccharification method, there are known a method using concentrated sulfuric acid, a method using diluted sulfuric acid and enzyme, and a hydrothermal saccharification method; however, there are still many problems to be solved in order to produce bioethanol at a low cost.
Meanwhile, there is a method in which cellulosic biomass is converted to a mixed gas containing hydrogen and carbon monoxide, from which ethanol is synthesized. With this method, an attempt is made to efficiently produce bioethanol from cellulosic biomass to which the application of ethanol fermentation is difficult. In addition, raw materials which can be used in this method are not limited to the wood-based biomass and the plant-based biomass, but also include various organic materials such as animal biomass derived from carcasses or feces of animals, garbage, waste paper and waste fiber.
As a method for obtaining an oxygenate such as ethanol, acetaldehyde or acetic acid from a raw material gas containing hydrogen and carbon monoxide, for example, there is known a method in which the raw material gas is contacted with a catalyst comprising rhodium, an alkali metal and manganese (see, for example, Patent Document 1).
Further, as a method in which a raw material gas generated from biomass is converted to ethanol, there is proposed an ethanol production method comprising a step of removing a sulfur-containing compound from a raw material gas (see, for example, Patent Document 2).